1. Field of the Invention
The present invention relates to a semiconductor device, a display device, and an electronic device.
2. Description of the Related Art
A shift register circuit is a circuit which operates such that content thereof is shifted by one stage each time a pulse is applied. Utilizing this property, shift registers are used in circuits for mutual conversion of serial signals and parallel signals. Such circuits which convert serial signals to parallel signals, or convert parallel signals to serial signals are mainly used for networks having circuits connected to each other. The number of transmission paths for connecting circuits to each other and transmitting signals in a network is often small relative to the amount of data to be transmitted. In such cases, parallel signals are converted into serial signals in a transmitter circuit and are sequentially sent to a transmission path, and serial signals that have been sequentially sent are converted into parallel signals in a receiver circuit. Thus, signals can be exchanged using a small number of transmission paths.
A display device display images by controlling the luminance of each pixel in accordance with image signals inputted from the outside. Here, since it is difficult to use a number of transmission paths of image signals from the outside that is equivalent to the number of pixels, it is necessary to subject the image signals to serial-parallel conversion. Therefore, shift registers are used in both a circuit which transmits image signals to the display device and a circuit for driving the display device which receives the image signals.
A CMOS circuit combining an n-channel transistor and a p-channel transistor is usually used in the above-described shift register circuit. However, in order to form a CMOS circuit combining an n-channel transistor and a p-channel transistor over the same substrate, it is necessary to form transistors having conductivity types opposite to each other over the same substrate, so the manufacturing process inevitably becomes complex. Consequently, increase in cost or decrease in yield of semiconductor devices results.
Therefore, a circuit having transistors which all have the same polarity (also referred to as a unipolar circuit) has been devised. A unipolar circuit makes it possible to omit some of the steps in the manufacturing process, such as the step of adding an impurity element. Thereby, increase in cost and decrease in yield can be suppressed.
For example, consider the case of forming a logic circuit in which all the transistors have n-channel polarity. This circuit has a problem in that when a potential with a high potential power supply is outputted, in accordance with the threshold of the n-channel transistors, voltage of an output signal is attenuated compared to voltage of an input signal. Therefore, a circuit called a bootstrap circuit is widely used so that the voltage of an output signal is not attenuated. A bootstrap circuit is realized when the gate electrode of a transistor capacitively coupled with an output terminal is floated after a transistor connected to the high potential power supply is turned ON so that current begins flowing through a channel. Thus, the electric potential of the output terminal rises and the electric potential of the gate electrode of the transistor also rises correspondingly, so as to eventually exceed the potential of the high potential power supply plus the threshold voltage of the transistor. Thereby, the potential of the output terminal can be made almost equivalent to the potential of the high potential power supply.
Using such a bootstrap circuit, a semiconductor device in which output potential is not attenuated even in the case of using a unipolar transistor can be realized. Further, a shift register circuit can be formed using the bootstrap circuit (for example, Reference 1: Japanese Published Patent Application No. 2002-215118 and Reference 2: SID2005, p.1050, “An Improved Dynamic Ratio Less Shift Register Circuit Suitable for LTPS-TFT LCD Panels”).